1. Field of the Invention
The present invention relates to a light-interference measuring apparatus.
2. Description of the Related Art
Conventionally, there is known a light-interference measuring apparatus such as a three-dimensional shape measuring apparatus. The three-dimensional shape measuring apparatus precisely measures the three-dimensional shape of a measuring object, for example, by using the luminance information of interference fringes generated by the interference of lights. A technique using a broad band light (white light and the like) as a light source is widely known in this light-interference measuring apparatus (see, for example, Japanese Patent Application Laid-Open Publication No. 2003-148921).
FIGS. 9A and 9B show an example of interference fringes obtained by using the broad band light as the light source. FIG. 9A shows the luminance of interference fringes of respective wavelength and FIG. 9B shows the luminance of superposed interference fringes.
As shown in FIGS. 9A and 9B, when the broad band light is used as the light source, the peaks of luminance of the interference fringes of the respective wavelengths overlap one another and the luminance of the overlapped interference fringes becomes larger at the focused position. However, the more distant from the focused position, the larger the maximum luminance positions of the interference fringes of the wavelengths shift from each other and the amplitude of the luminance of the superposed interference fringes be smaller gradually.
Therefore, the light-interference measuring apparatus can consequently measure, for example, the three-dimensional shape of a measuring object by detecting the position of peak luminance at each position in a visual field.
The interference objective lenses to be used for such light-interference measuring apparatus mainly include Michelson type lenses and Mirau type lenses, which are used according to the magnification ratios of the interference objective lenses. In general, the Michelson type lenses are used for the interference objective lenses of low magnification ratios, and the Mirau type lenses are used for the interference objective lenses of high magnification ratios.
FIG. 10 is a schematic view showing the basic configuration when the Mirau type interference objective lens is used.
As shown in FIG. 10, in the case where the Mirau type interference objective lens is used, an optical path of light emitted from the interference objective lens is branched by a beam splitter into a reference optical path (denoted by the broken line in the drawing) including a reference mirror therein and a measuring optical path (denoted by the solid line in the drawing) including a measuring object arranged therein. Thereafter, reflected light from the reference mirror (reference light) and reflected light from the measuring object (object light) are superposed.
At the focused position, that is, when the optical path lengths of the reference optical path and the measuring optical path are equal to each other, here, the respective lights become light waves having same phase with each other as shown in FIG. 11A.
Therefore, the respective light waves of the reference light and the object light reinforce each other at the focused position and become an interference wave as shown in FIG. 11B. A condition of actual interference fringes in such an optical system is shown in FIG. 11C. In FIG. 11C, a bright zero-order interference fringe is observed at the focused position, and bright first-order interference fringes are observed around the focused position.